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Categorization of stars
Categorization of anything has been very important for scientific studies. It makes it easier to understand properties. Most of us might have studied the periodic table of elements. How easy it becomes studying them? However, categorizing stars were very hard. By just seeing from the earth, you can’t tell, about the size of a star, its temperature, and its luminosity. So, we have different methods to find out these features of stars.
Estimating Stellar radii
The main method to find the radius of a star is by using Stefan-Boltzmann’s law or the radiation law:
Which states that the luminosity per unit area of a body is directly proportional to the fourth power of its surface temperature i.e.,
By the above relation, we can get the surface area and ultimately the radius of a star.
NOTE: there are several methods to determine the distance of the star from the earth. We will discuss them in another article. To determine the luminosity the distance of the star must be known.
Finding surface temperature
To use Stefan-Boltzmann’s law, we must have the value of the surface temperature of a star.
We can find this with the help of the black body curve. It is a graphical relationship between the wavelength of the highest intensity light emitted by a star and its surface temperature. This curve is so well understood by Astronomers that they only use only two filters, B (blue) and V (visual- for green/yellow) to get two specific ranges of light and determine the star’s surface temperature.
spectroscopy
By observing the absorption spectrum, we can determine the constituents of a star. In this method, the light emitted by a star is absorbed (a very tiny fraction) by its constituents. The spectrum of these lines is not continuous. Gaps are specific to the substance that absorbs it therefore we can determine the constituents of stars by this method. It also helps to find the age of a star and how long it will live.
Classification
By using the data of the above equations now we can classify stars into the groups
spectral classification
The main classification is the spectral classification which is based on a star’s surface temperature and its absorption line. There are different spectral classes named O, B, A, F, G, K, and M, they are in descending order of temperature. O type star corresponds to a star having a temperature near 30000 K. There is a method to remember the sequence of spectral classes “Oh Be A Fine Girl/Guy, Kiss Me”.
There is also a subdivision from 0-9. The lower the number hotter the star will be. For example, our sun, whose surface temperature is around 5800K, is a G2-type star (a little hotter than G3 and a little cooler than G1). One of the most luminous stars of constellation ORION, Betelgeuse is an M2-type star. You can find the Betelgeuse on the head of the Orion.
There are differences in the color of stars, the hotter will be the surface, the higher will be the frequency of light emitted by the star. The O-type star corresponds to an electric blue color. One example of the O-type star is Mintaka (The star on the right side of the Orion belt).
Based on size
Stars are found in different sizes. Most of the stars are from 0.1-10 times the solar radius. The stars from 10-100 times larger than the sun, like Aldebaran, are called giants. If a star is larger than that then it’s a supergiant. More specifically, if they are having less temperature than the sun, then it’s red giant/supergiant. The Betelgeuse is a red supergiant and Rigel is a blue supergiant, and both lie in the Orion constellation.
Similarly, stars smaller than the solar radius (including the sun itself) are known as dwarfs.
| SPECTRAL CLASS | TEMPERATURE (K) | PROMINENT ABSORPTION LINES | COLOUR | FAMILIAR EXAMPLES |
| O | 30,000 |
Ionized helium (strong); multiple
y ionized heavy elements; hydrogen (faint) |
Electric blue | Mintaka (O9) |
| B | 20,000 | Neutral helium (moderate); singly ionized heavy elements; hydrogen (moderate) | Blue | Rigel (B8) |
| A | 10,000 | Neutral helium (very faint); singly ionized heavy elements; hydrogen (strong) | white | Vega (A0), Sirius (A1) |
| F | 7,000 | singly ionized heavy elements; hydrogen (moderate); neutral metals | Yellow-white | Canopus (F0) |
| G | 6,000 | singly ionized heavy elements; hydrogen (relatively faint); neutral metals | Yellow | Sun (G2), Alpha Centauri (G2) |
| K | 4,000 | singly ionized heavy elements; hydrogen (faint); neutral metals(strong) | orange | Arcturus (K2), Aldebaran (K5) |
| M | 3,000 | Neutral atoms (strong); molecule (moderate); hydrogen (very faint) | red | Betelgeuse (M2), Barnard’s star (M5) |
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